1. Field of the Invention
This invention relates to a catalytic oxidation chamber and method of its use, particularly in minimizing energy consumption and the volume of waste exhaust, while treating a wide range of concentrations of air pollutants from both batch and continuous processes. The method and device of the present invention is particularly well suited to control of ethylene-oxide emissions from a gas sterilizer.
2. Brief Statement of the Prior Art
Catalytic oxidizers are commonly used in controlling atmospheric emissions. These oxidizers employ a catalyst within an oxidation chamber, which is typically filled with a granulated metallic oxide or a mixture of oxides, or an inert substrate such as alumina, over which a precious metal such as silver or platinum has been deposited. Regardless of which catalyst is used, proper operation of the oxidation chamber is critical. The constituent to be oxidized is typically referred to as a contaminant after it has been used for its central purpose. These contaminants are typically one or more combustible gasses. Most importantly, the total concentration of gaseous contaminants, hereafter combustible gasses, in the process airstream to be oxidized must not exceed a limit which is determined by the nature of the catalyst. This limit is generally set at 15 percent of what is referred to as the lower explosive limit, or L.E.L. When this lower explosive limit is exceeded, the heat of oxidation accumulates to excess, and the catalyst bed can overheat, unless special provisions have been taken to absorb this heat.
The most common method of preventing overheating of the catalyst bed is to add uncontaminated atmospheric "dilution air" to the process airstream entering the oxidation chamber, to lower the lower explosive limit and adsorb a portion of the heat released in the catalyst bed during oxidation. One Japanese Patent, No. 12286 to K. K. Zeon demonstrates this practice. The dilution-air stream is even identified as such in the patent abstract. Another method of preventing overheating in the catalyst bed is to place an inert solid material within the bed to absorb a portion of the heat released during oxidation.
To date, it has not been economically feasible or even desirable to use heat-absorbing material as the sole means of preventing overheating of the catalyst bed, using no atmospheric dilution air at all, due to the continuous nature of most processes and the enormous quantities of heat to be absorbed.
The use of heat-absorbing materials in oxidizes is generally limited to use in regenerative heat-recovery exchangers, which work by reversing the direction of air flowing through them. Examples include U.S. Pat. No. 4,702,892 to Erwin C. Betz, entitled "Heat Recuperative Catalytic Oxidation Device", and U.S. Pat. No. 4,770,857 to Gerhard Ludwig, entitled "Process and Apparatus for the Catalytic Reaction of Gasses".
A less frequent use of heat-absorbing materials includes the retention of the heat of oxidation to slow the cooling of the catalyst bed, when the process air flow and supply of oxidizable combustible gasses stops. Examples include U.S. Pat. No. 3,598,543 to Howard M. Crosby, entitled "Catalytic Exhaust Purifier", and U.S. Pat. No. 3,874,854 to Joseph E. Hunter, entitled "Catalytic Converter".
In addition to limiting the maximum concentration of oxidizable contaminants in the process airstream entering the oxidation chamber, a second critical operating variable is residence time, or the time it takes for the process air stream to pass through the catalyst bed. This is achieved by using the proper combination of process air flow rate and depth of catalyst bed. A third critical operating parameter is the operating temperature of the catalyst bed. It must be greater than the minimum temperature needed to obtain the desired degree of oxidation or control efficiency, yet it must be lower than the temperature at which the catalyst is damaged. Traditionally, the temperature of the catalyst bed is generally controlled by using the proper ratio of dilution air which has the effect of cooling both the bed and dissipating any heat available from a process air heater employed to heat the bed.
Optimum performance of an oxidizer is obtained when the optimum combination of residence time and catalyst bed temperature, is achieved for a given concentration of oxidizable contaminant in the process airstream. In practice, this optimum combination, known as the time-temperature relationship is very difficult to achieve. When an oxidizer, including its oxidation chamber is put into service, any change in process air flow rate or concentration of oxidizable contaminant upsets the time-temperature relationship. The present state of the art requires control of the temperature of the oxidation chamber, especially in response to changes in process air flow rate or contaminant concentration, so that the desired degree of emission control or oxidation efficiency is achieved, even at the expense of wasted energy.
Changes in residence time in the catalyst bed are generally not possible, as the depth of the catalyst bed is fixed, and the process air flow rate cannot be easily changed without creating problems in the process from which the air is flowing. Often, the only variable which can be controlled is the process air temperature, even when this does not achieve optimum results.
One disadvantage to using catalysts to enhance oxidation of combustible gases is the susceptibility of the catalyst to poisoning and masking. Poisoning is the reaction of a contaminant with the catalyst itself. Masking is the deposition of a contaminant onto the surface of the catalyst, blocking the availability of reactive sites at the surface. Both phenomena reduce the effective active surface area of the catalyst and make it less active, or effective. The current state-of-the-art methods utilized in catalytic oxidation control both poisoning and masking by preventing a gaseous contaminant, capable of causing either effect, from entering the process air stream entering the oxidation chamber.
In the event either poisoning or masking occurs, the catalyst will lose activity and it must be removed. The catalyst is either reinstalled after cleaning, or replaced with new catalyst, to restore the efficiency of the oxidizer.
Catalytic oxidizers have been used for many years, and at various types of facilities, as air-pollution control devices. A typical control efficiency set by regulatory agencies for the control of smog-forming chemicals is 95 percent. Recently, regulatory agencies have adopted standards for the control of toxic air emissions which are known or are suspected of causing cancer. These regulations are much stricter than those used to control smog-forming chemicals, requiring up to 99.99 percent control of emissions passing through the oxidizer, and they require that the total mass of emissions released be limited to a designated amount.
The mass or weight of emissions released from an oxidizer is calculated by simply multiplying the exhaust gas flow rate times the concentration of gas contaminant in the exhaust. As the practical limit on reducing the concentration of gas contaminant in the exhaust is reached, the only way to lower the weight of emissions released into the atmosphere, is to lower the total exhaust gas flow rate. Because the regulations for controlling air toxics are relatively new, little work has been performed in minimizing exhaust flow rate. The oxidizer presented here is designed specifically to minimize exhaust gas flow rate.
It is difficult to design a catalytic oxidizer specifically for controlling the emissions from a batch process. Often, both the flow and concentration of combustible gasses vary during the operating period of the process. When cooling of the catalyst bed is achieved through dilution air, the flow of dilution air must be continuously adjusted, for optimum operation. This is difficult and expensive to accomplish and is therefore generally not practiced.
As an example of this difficulty, existing Catalytic oxidizers are not well suited to controlling emissions from gas sterilizers, which operate as a batch process. Gas sterilizers use a toxic sterilizing gas which contacts the objects to be sterilized. After sterilization, the toxic sterilizing gas must be safely removed, reacted to eliminate its toxicity, and vented. Catalytic oxidizers currently used to control emissions from gas sterilizers operate with fixed dilution-air flow rates, selected to keep the oxidation chamber small and inexpensive. The small-sized oxidation chamber and fixed dilution-air flow therefore limits oxidation capacity. This limited capacity of the oxidizer retards the normal rate of venting of the gas sterilizer and lengthens the total time required to operate the gas sterilizer.
In the case where exhaust gases are recirculated to conserve energy in catalytic oxidation, the reacted exhaust gases are typically mixed back into the gaseous contaminant to be oxidized. This mixture is then passed through the catalyst bed. The state-of-the-art method currently used for mixing the exhaust gas with the incoming gaseous contaminant is to use an empty chamber or section of ductwork for the mixing. The degree of mixing is limited by the lack of turbulence in these mixing schemes, and the mixture is not truly homogeneous and uniform. As a result, the intensity of the oxidation process, which is dependent upon the concentration of oxidizable contaminant, varies across the surface of the catalyst bed. The temperature of the catalyst is not uniform, and optimum efficiency of the bed is not achieved.
The state of the art used to overcome this less-than-optimum catalyst efficiency is to use more catalyst, or to operate the bed at a higher temperature. Either approach is wasteful and increases the operating cost of the oxidizer.
Specifically, sterilizers are commonly used in hospital and food applications. These sterilizers have a chamber which is sealed and filled with a sterilant gas mixture containing from 10 to 100 weight percent ethylene oxide. Commonly, the ethylene oxide is diluted with an inert gas such as one of the various chlorofluorocarbons, e.g., Freon 12, or carbon dioxide, to create a nonflammable gas mixture.
In practice, the objects to be sterilized are placed in the chamber, the chamber is sealed and filled with the sterilant gas for a sufficient time to permit penetration of the ethylene oxide throughout the objects and effect complete sterilization. Once the sterilizing has been completed, the sterilant gas is pumped from the sterilizing chamber by a vacuum pump. When the exhaust from the vacuum pump is vented to the atmosphere, it contributes significantly to air pollution, as ethylene oxide is a very toxic gas which is known to cause cancer and is objectionable in the environment. In some areas of the country, notably Southern California, regulations prohibit direct venting of ethylene oxide to the atmosphere. In such areas, ethylene oxide emissions must be controlled. This can be accomplished by passing the sterilant gas mixture containing ethylene oxide mixed with air over an oxidation catalyst at temperatures sufficient to oxidize ethylene oxide to carbon dioxide and water vapor.
All the catalytic oxidation systems which have been employed previously to control ethylene oxide emissions mix the ethylene oxide with a constant flow of air, thereby controlling the temperature of the catalyst bed. The air added to the ethylene oxide is a heat sink for the heat of oxidation of the ethylene oxide. The more concentrated the ethylene-oxide emission, the more air must be added to control the temperature rise in the catalyst bed, to prevent overheating and thermal damage to the catalyst, and to prevent thermal breakdown of the chlorofluorocarbon, should it be present as a diluent gas.
The existing catalytic oxidation systems are not ideally suited for the oxidation of a fixed quantity of ethylene oxide such as is encountered in venting of sterilizing chambers. As the sterilizing chamber is vented and fresh air is subsequently washed into and through the sterilizing chamber to remove residual ethylene oxide, the concentration of ethylene oxide in the air stream entering the catalytic oxidizer continuously declines. Consequently, the catalyst is never provided with a gas mixture having a constant concentration of ethylene oxide. This results in serious compromises in the design of the oxidizer which is designed to treat a relatively large volume of air, dictated by the initial high concentration of ethylene oxide vented from the sterilizing chamber. Except for the beginning of the venting of the sterilizing chamber, existing catalytic oxidizers are oversized and are very inefficient in their use of energy. Even with the attachment of heat recovery devices, the inefficiency of heating relatively large volumes of air to treat decreasing concentrations of ethylene oxide cannot be overcome.